Real time mirroring of a selection of objects?
Take all the objects that you want mirrored and add them to a Group. (Select them and use Ctrl-g).

Then create a single empty somewhere on the mirror axis, and scale its x or y axis by -1 (I edited the scale value in the properties but s x -1 would also work.  Now in the Properties > Object panel in the Duplication section change its duplication from None to Group and select the group you created in the first step.  There should be copies of all the objects mirrored along the axis that was scaled by -1.

When you create more objects that need to be part of the mirror group you can go into the Properties > Object panel and look for the Groups section and use the Add to Group button to add it to the dupligroup.